The field of this invention is manipulation of small volumes comprising a volatile liquid.
Microfluidic devices comprise small capillary channels in a solid substrate, where the channels are usually present as a network. Various orifices are provided for communicating with the channels. Because of the small volumes of the networks and the individual channels many benefits adhere. The small volumes require less reagent and sample, frequently being limited by the level of detection available. In addition, because of the small volumes, reactions are very rapid. The networks allow for efficient movement of the components from one site to the next and with little loss of the components. Also, various components may be brought together, separated by different operations and the individual fractions used for various purposes.
The microfluidic devices lend themselves for various assays involving candidate compounds, where binding events are measured, enzyme activity measured, or metabolic processes measured. In this way, the effect of the candidate compounds on the indicated events may be determined. Where one is interested in comparing the effect of different candidate compounds, it is necessary that the amount of the candidate compound and other solutions that will be used are aqueous. Unless one uses relatively drastic measures, the water will rapidly evaporate. Transfers of aqueous or other solutions involving manipulative steps where the solution is exposed to the atmosphere for any length of time will invariably result in some evaporation, particularly where there are sequential additions, and the solvent from the earlier additions is evaporating while adding the next addition and during the interim between additions. In addition, incubations can result in evaporation, even where the container is covered. The problem is exacerbated where one is interested in high throughput screening, which may involve many very small aliquots of different solutions to multiple sites on a microfluidic device. Using foreign substances to diminish the evaporation can lead to contamination, require repetitive cleaning and create other detrimental issues.
Various methods have been tried, such as cooling the liquids, so as to substantially reduce evaporation, adding a lower volatility liquid over the surface of the sample, ambient humidity, adding droplets of solvent to the sample after its deposition to maintain the volume, and the like. All of these approaches are not generally useful and have severe disadvantages for use with small volumes, which must be transferred to a reaction vessel. There is a need for improved methods for manipulating nanoliter volumes when dealing with microfluidic devices, particularly associated with high throughput screening of compounds, diagnostic assays or other investigative procedures.
U.S. Pat. Nos. 5,576,197 and 5,282,543 disclose the use of wax and other flexible materials, respectively, to inhibit evaporation. Microfluidic devices are described in U.S. Pat. Nos. 5,885,470; 5,858,195; 5,750,015; 5,599,432; and 5,126,022. Methods of evaporative control are disclosed in WO98/33052 and WO99/34920.
Methods and devices are provided for the manipulation of small volumes in association with determinations employing microfluidic devices, where a substantial portion of the liquid is subject to evaporation during the operation. The microfluidic devices comprise a partial enclosure for a zone for receiving a small amount of a component of the operation, usually as a solution comprising a component of a reaction. The zone is bounded by a meniscus, whose position is affected by the nature of the zone, which zone may have a non-wettable border, which may be made wettable by addition of a detergent or may be wettable. During the operation, the liquid in the zone is subject to evaporative loss of liquid, and the zone is in fluid exchange relationship with a channel housing a replenishing liquid. The channel liquid replenishes the liquid in the zone and may serve as a source of a second or more components of the operation. During the operation, the position of the meniscus will be relatively fixed in a number of embodiments, while in other embodiments be subject to the movement of liquid into and out of a capillary channel. Either substantially immediately upon entering the zone, the component is in contact with the channel liquid, so that any solvent lost by evaporation in the zone can be replenished, or the component is placed at a site where evaporation of any liquid may occur and the residue is dissolved in a liquid discharged from a capillary channel, where contact is maintained with the solution which forms the zone and the solution in the capillary channel. The reaction volume is substantially maintained in the zone defined by a major portion of the components of interest being present in the zone, comprising the region between a meniscus and the region of liquid exchange between the zone and the channel.